The latest stages of mammalian gestation entail an accelerated growth phase, which is critical for optimal neonatal birth weight and preparedness for life outside the womb. While it is apparent that the fuel source for this massive growth spurt is circulating maternal metabolites, it is less clear whether placental storage of metabolic fuels is essential for this process. Insights into this process emerge from our studies of the nuclear receptor PPAR?, an essential regulator of placental development and metabolism. We found that the placenta- enriched transcription cofactor LCoR (Ligand-dependent CoRepressor) is a coactivator (Not a corepressor) of PPAR?, on the target gene Mud both in vitro and in vivo. Molecular genetic analyses revealed that LCoR- deficient neonates die within hours of birth. These neonates do not exhibit obvious anatomic anomalies, but display limp forelimbs, lethargy, and failure to suckle. Importantly, while exhibiting normal growth until mid- gestation, Lcor-null embryos are significantly growth-restricted at term, alongside significant enlargement of the placenta. Histological analyses reveal that Lcor-null placentas retain a substantially higher number of placental glycogen trophoblasts (GlyT) compared to their WT counterparts, providing a plausible explanation to their larger mass. Importantly, both PPAR?, and LCoR are critical for placental expression of the glucagon receptor gene, Gcgr, suggesting that Lcor-null placentas accumulate glycogen due to failure to respond to the glycogenolytic signal of glucagon. Our findings suggest that LCoR regulates export of glycogen-derived fuel from placental GlyT to the embryo, and that its loss restricts the growth of the late fetus by limiting its access to this fuel. We further surmise that this severe deprivation of placental fuel sources at the last stages of gestation underlies the fatal, lethargic state of Lcor-null neonates. Accordingly, our overarching hypothesis is that transcriptional and developmental regulation of placental glycogen stores plays an essential role in late embryonic growth and neonatal survival. Three specific aims driven by this hypothesis will dissect the role and regulation of placental glycogen stores in late embryogenesis: Aim 1. Determine the outcomes of glycogen trophoblast ablation. Aim 2. Define the lineage-specific functions of LCoR in the placenta and the embryo. Aim 3. Dissect the molecular mechanisms of placental gene regulation by LCoR and PPAR?.